1. Field of the Invention
The present invention is directed generally to digital signal processing, and in particular to receiver based methods and devices for combating co-channel NTSC interference in digital HDTV transmission.
2. Description of the Prior Art
The Federal Communications Commission and cable television testing organizations such as CableLabs have been evaluating digital television delivery systems in order to choose a new television "standard" which someday will replace NTSC in the United States. These systems all involve digital coding and data compression techniques, for example those utilizing the MPEG algorithms or variations thereof.
The FCC plans to test and approve an advanced television (ATV) standard comprising for example, high definition television (HDTV) and standard definition television (SDTV) digital signals for terrestrial broadcasting. Although the specifics of the standard are yet to be fully tested and agreed upon, the FCC has indicated that the system will initially take the form of a so called "simulcast" approach. The new ATV signals will fit into currently unused television channels (so-called "taboo" channels) and initially co-exist with the conventional analog television signals without co-channel interference.
An example of a taboo channel is as follows: FIG. 1 shows two geographical areas A and B. Transmission antenna 1 in area A transmits NTSC channel 6. The FCC will not permit antenna 2 in area B to also transmit NTSC channel 6, as the proximity of antenna 2 to antenna 1 could cause NTSC interference in both areas. Accordingly, if area A is transmitting NTSC channel 6, then channel 6 in area B is a so-called "taboo" channel. Under the planned ATV standard however, The FCC will now permit antenna 2 in area B to transmit ATV signals on channel 6 as the new ATV signals are much more resistant to NTSC interference, although NTSC interference will still be a problem especially in the fringe area 3.
NTSC will be used hereinafter to represent one example of conventional television broadcasting. Other examples would be, inter alia, SECAM and PAL. Although NTSC is exemplified herein, it is not meant to be construed as a limitation and will be used herein synonymously with "conventional" to represent conventional television in general.
The FCC intends to test the so-called "Grand Alliance" digital ATV system, a system which is being cooperatively developed by the corporate sponsors which developed the first round of individual proposals which were tested by the FCC in 1991 and 1992.
The Grand Alliance has already decided on a coding algorithm which will comply with the source coding standards proposed by MPEG (Motion Pictures Experts Group). In addition, the RF transmission scheme selected on the basis of testing conducted by the Advanced Television Test Center (ATTC) is the trellis coded 8VSB system designed and built by Zenith Electronics. The system details are described in "Digital Spectrum Compatible--Technical Details", Sep. 23, 1991 and more recently modified for bakeoff and described in "VSB Transmission System: Technical Details", Sep. 19, 1994, which are incorporated by reference herein. This system is also described in an article in Broadcast Engineering by Chan, entitled "A Look at 8-VSB", September 1994 at page 10, and in TV Technology in an article by Weiss, entitled "And the Winner is VSB. Maybe", April 1994 at page 50 which are hereby incorporated by reference.
The technique used in the Zenith 8VSB modem to combat this co-channel interference is as explained below with reference to FIG. 2.
FIG. 2 shows an example of the frequency spectrum of an NTSC signal. The carriers of the picture 5, sound 6 and color 7 of the NTSC signal will cause interference with the digital HDTV signal. In order to remove these "peaks" where the carrier frequencies of the co-channel lie, the Zenith 8VSB modem uses a comb filter in the receiver having a response 4 which introduces nulls, in the digital spectrum at the locations of the carriers of the picture 5, sound 6 and color 3 as shown. The comb filter, shown in FIG. 7, provides rejection of steady-state signals located at the null frequencies. In order to ensure that the nulls align with the correct carrier frequency, the ATV spectrum must be shifted 45.8 khz with respect to the NTSC spectrum, as described at page 15 of "VSB Transmission System: Technical Details", and also delayed by 12 symbols, as described below.
FIG. 4 shows the prior art use of the comb filter in the Zenith system. The comb filter is switched into use when NTSC interference is present in the digital signal, otherwise the comb filter is not used. This provides significant improvement in the performance of the digital signal when a conventional television signal, e.g. NTSC, is broadcast on a co-channel. A general description of a comb filter can be found in Pohlmann "Principles of Digital Audio", 2nd ed. 1991, hereby incorporated by reference.
The comb filter in the prior art is treated as a partial response channel in cascade with a trellis coder, for the case when co-channel conventional television interference is present. An optimum decoder can then be developed which uses Viterbi decoding on an expanded trellis, the states of which correspond to the cascade of the states of the comb-filter and the trellis coder as described in "Principles of Digital Communication and Coding" by Andrew J. Viterbi and Jim K. Omura, 1979 (hereby incorporated by reference) specifically at pages 227-300. The number of states in the trellis becomes increasingly large as the number of delays in the comb filter increase. Accordingly, the trellis coded 8VSB system, which uses a comb filter with a delay of 12 symbols, has an extremely large number of trellis states and also suffers from the problems associated with additive noise.
To simplify their design, Zenith converts the MPEG coded and RS coded and interleaved data-stream from serial to parallel, then uses twelve parallel trellis encoders followed by a parallel to serial converter at the transmitter. Thus now the trellis decoder for the case when the comb filter is used, implements Viterbi decoding on a trellis with the number of states equal to two or four times the number of states of the trellis encoder. The problems associated with additive noise still occur and in fact cause the trellis decoder to work improperly.
When the co-channel conventional television interference is absent, the comb filter is not switched into use and the Viterbi decoding is implemented on a trellis with the number of states equal to the number of states of the trellis encoder. This is possible since, with the Zenith approach, pre-coding is not used in the transmitter. The choice between the path 10, afforded by simple trellis decoding, or of the path 20, which employs the comb filter and the expanded trellis at the receiver, is decided by the measured error-rate of the periodically sent data field sync symbols at the outputs of the post comb and with no post comb. Thus, when there is NTSC interference the comb filter path 20 will be chosen which increases the number of states of the trellis decoder. The switching between the use of a comb filter in the receiver or not, suggested by Zenith, is cumbersome. In addition a significant number of computations must be performed to determine whether the comb filter should be used or not. Also the use of the comb filter requires 12 parallel encoders and correspondingly 12 parallel decoders which is also very cumbersome.
Another problem associated with the present comb filters is that when both NTSC co-channel and all white gaussian noise (AWGN) are present the performance of the comb filter degrades dramatically. This is because the AWGN, after passing through the comb filter, does not remain white, but gets "colored", that is, the noise samples are no longer independent of each other. This "colored" noise affects the performance of the trellis decoder which is optimized for performance in an AWGN channel. Since the co-channel conventional television interference is maximum at the fringe area 3 in FIG. 1, where the signal power is small and hence the AWGN is large, this "coloring" of the AWGN is indeed a scenario which must be taken into account.
The use of a comb filter for NTSC rejection also requires that the ATV spectrum be shifted 45.8 khz with respect to the NTSC spectrum in order to align the nulls of the comb filter with the picture and color carriers as described in "VSB Transmission System: Technical Details", at page 15. This causes the digital spectrum to spill over into the adjacent 6 Mhz channel which is undesirable for adjacent channel rejection.